1. The ISM and Stellar Birth

2. Extinction and Reddening
Rayleigh Scattering

3. Extinction and Reddening

5. Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs
Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs. That is, a star
1000pc from Earth will look about 2 magnitudes fainter than if space were empty completely

6. Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs
Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs. That is, a star
1000pc from Earth will look about 2 magnitudes fainter than if space were empty completely
Dust thought to come from stellar ‘winds’, blowing out molecules of hydrogen, carbon, oxygen
and other elements which cool and coalesce into dust grains
Carbon IR
Visible UV

7. 2. Nebulae Emission Nebulae: Light is from emission spectrum
Reminder: result of a low density gas excited to emit light. The light is emitted at specific wavelengths
The gas is excited by light from hot stars > 25,000K (B1). It does not shine under it’s own light.
Sometimes called HII regions, as they mostly contain hydrogen that has been ionised by the light from stars
Density: atoms per cubic cm
Pink due to red, blue and violet Balmer
emission lines

8. Orion Nebula (M42)
Eagle Nebula

9. Reflection Nebulae:
Light reflected (scattered) by dust/gas much like the moon
reflects the Sun’s light – so doesn’t generate its own light
Scatter light from cooler stars
Mostly scatters blue light (like our atmosphere) – so they appear blue.
Dust grains must have sizes ranging from 0.01mm down to 100 nm
See absorption spectrum of nebula in the star’s spectrum
Doppler broadening due to motion of gas molecules
Lines split into more than one component indicates light travelled through
different gas/dust clouds with different radial velocities

10. Witch Head Nebula
Merope Nebula

11. Dark Nebulae:
More dense clouds of dust and
gas obscure light from background
stars
Very cool ( K)

12. Horse’s head nebula
Snake nebula

14. The components of the Interstellar Medium (ISM):
HI clouds
seen through interstellar absorption lines and
21 cm radio radiation
Neutral Hydrogen
50 – 150pc diameter
Few hundred K
10 – 100 molecules / cubic cm
Twisted into long filaments
Near Stars, it is ionized to form
HII regions

15. The components of the Interstellar Medium (ISM):
Hot intercloud medium
Between HI clouds
Few thousand K
0.1 molecule / cubic cm
Mostly hydrogen (HII) ionized by ultraviolet light from
stars

16. The components of the Interstellar Medium (ISM):
Giant molecular clouds (GMCs)
Contain larger molecules, sometimes organic, although
still mostly hydrogen
10K
1000 – 100,000 molecules / cubic cm
15 – 60pc across
Often seen as dark nebulae

18. Our solar system .

19. The components of the Interstellar Medium (ISM):
Coronal gas
100,000 – 1,000,000 K
0.001 – atoms / cubic cm
Ionized atoms
From supernovae or very hot stars
Emit X-Rays
Nothing to do with the Sun’s corona!

21. We see evidence for the interstellar medium through...
Extinction and reddening
Emission nebulae
Dark nebulae
Reflection nebulae
21cm radiation
X-rays from hot gas between stars
...from which we can figure out the components of the ISM:
HI clouds
Hot Intercloud medium
Giant molecular clouds
Coronal gas

22. Stars are born when a small part of a giant molecular cloud collapses
Resistance to collapse:
Heat energy
10K: average speed of hydrogen
Molecule is 800mph
Magnetic fields – act as springs
3. Rotation
4. Turbulence

23. Need a triggering mechanism: shock waves

24. Need a triggering mechanism: shock waves
Shock wave from:
Supernova explosions
Ignition of hot nearby stars
Collision of molecular clouds
Spiral pattern of galaxy

25. NGC 1999 – Reflection nebula containing a small clump of a giant molecular cloud
collapsing to form stars

26. Protostars Clumps of compressed gas resulting from
the shock wave passing through the gmc
start to collapse under their own gravity
As gas molecules fall in, their speed increases
They collide with other molecules and
randomize their speeds
Temperature is just a measure of how fast, on
average, the random motion of molecules is
So as the cloud collapses, its temperature
increases

27. Protostars Protostar Dust Free Zone IR photon
Outer Envelope cloaks protostar of Gas and Dust

28. Protostars
As cloud collapses, it flattens out into a disk due to rotation

30. Protostar continues contracting and heating up until the center becomes hot enough
to start fusing hydrogen into helium > the star is born!
Drive away their cocoon of dust and gas

32. Birth line
Hayashi track